In recent years, artificial muscles of such configuration that air is injected into a hollow elastic body to expand (or inflate) it thereby contracting it in the longitudinal direction have been known. FIG. 9A is an illustration showing a structure of a McKibben type artificial muscle 50 which has hitherto been under study. The artificial muscle 50 has a structure of a cylindrical rubber tube 51 covered on the outside by a sleeve-like braided fiber cord 52. The rubber tube 51 and the fiber cord 52 are strongly secured at both ends by terminals 53 and fastening bands 54. As the rubber tube 51 is expanded with air injected thereinto through an air injection pipe 55 provided in the terminal 53, the angle 2θ between fibers 52a and 52a of the fiber cord 52 changes as shown in FIG. 9B. And this causes the artificial muscle 50 to contract in the longitudinal direction. Hence, the artificial muscle 50 operates as an actuator with the distance between the terminals 53 and 53 changing.
Yet, this McKibben type artificial muscle 50, which consists of a rubber tube 51 covered with a fiber cord 52 only, has been subject to a problem of tearing rubber or the like because friction occurs between the rubber tube 51 and the fiber cord 52 at contraction and extension (or elongation).
Thus, a rubber artificial muscle 60 so configured that fibers are inserted in a rubber tube as shown in FIGS. 10A and 103 has been proposed. The rubber tube 61 of the rubber artificial muscle 60 has a plurality of kite strings (cotton yarn) 63 inserted and extending therein in the longitudinal direction which restrict the longitudinal extension of the rubber tube 61. In this arrangement, the kite strings 63 are in one piece with the surrounding rubber film 62. This helps improve the durability of the artificial muscle 60 because friction between fibers (kite strings 63) and rubber (rubber film 62) at the contraction and extension of the rubber tube 61 can be eliminated. (See Non-patent literature 1, for instance.)    Non-patent literature 1: Matsushita: Gomu Jinkokin Seisakuhou Noto (Notes on Fabrication of Rubber Artificial Muscle); “Keisoku To Seigyo” (Measurement and Control), Vol. 7, No. 12 (November 1968): pp. 110-116